Tunable lasers are used to generate laser light at different wavelengths. Such lasers can be employed in a variety of technical applications, such as optical communications, spectroscopy, and photochemistry, to name but a few. As one example, an optical communication system may use a tunable laser to transmit data through different communication charnels using wavelength division multiplexing.
A tunable laser typically comprises a cavity that recirculates and amplifies selected wavelengths of an optical field to produce a laser beam. The cavity typically comprises a wavelength filter that selects wavelengths to be amplified by the cavity and again element that amplifies the selected wavelengths. As examples, the wavelength filter can be implemented by a diffraction grating and the gain element can be implemented by a semiconductor optical amplifier (SOA).
The tuning range of the laser is generally determined by properties of the cavity such as its gain spectrum and cavity modes. The gain spectrum of the cavity defines a set of wavelengths that are amplified as the optical field recirculates. To produce lasing action, a wavelength must fall within the gain spectrum so that it is amplified during the recirculation. The gain spectrum is generally determined by properties of a gain element and any lossy elements in the cavity. The cavity modes, on the other hand, define a discrete set of wavelengths that resonate within the cavity based on their relationship to the cavity's optical length. To produce lasing action, a wavelength must belong to one of the cavity's cavity modes in addition to fatting within the gain spectrum. A wavelength belongs to a cavity mode if a round trip path through the cavity is equal to an integer multiple of the wavelength. Because the cavity modes relate to the optical length of the cavity, they can be modified by adjusting this length.
When a laser is tuned between different wavelengths, its output may exhibit discontinuities due to so-called “mode hops” between different cavity modes. For example, if the laser is tuned from a first wavelength corresponding to a first cavity mode to a second wavelength corresponding to a second cavity mode, the output of the laser may be interrupted as the tuning passes through intermediate wavelengths that do not belong to any of the laser's cavity modes. The laser can also experience discontinuities due to phase differences between successively output wavelengths. For example, if the laser is tuned from between successive wavelengths that are out of phase with each other, the resulting output may be distorted or undefined at certain points.
The discontinuities caused by mode hops or phase differences can hinder the performance of certain applications. For instance, in optical test instrumentation, it can be useful to perform a continuous wavelength sweep with a tunable laser to test an optical device's performance over a full range of wavelengths. However, these discontinuities generally prevent the sweep from being performed for all of the wavelengths, and they can introduce noise into the resulting measurements.
To avoid these and other limitations, researchers have developed lasers that can be tuned in a phase-continuous manner (i.e., without mode hops and in phase) by adjusting their cavity modes to compensate for changes in wavelengths. For instance, when a phase-continuous tunable laser is tuned from a first wavelength to a second wavelength, the cavity's optical length can be adjusted together with the selected wavelength in order to avoid mode hops and ensure that the second wavelength is output in phase with the first wavelength.
FIG. 1 is a diagram illustrating a conventional cavity that can be used to implement a continuously tunable laser. In the example of FIG. 1, the cavity has a Littrow configuration, but it could be modified to have other configurations, such as a Littman-Metcalf configuration.
Referring to FIG. 1, a cavity 100 comprises an optical amplifier 105, a collimating lens 110, and a diffraction grating 115. Optical amplifier 105 has a partially reflective mirror 125 at one end and an anti-reflective coating 130 at another end. An emission wavelength of cavity 100 is tuned by rotating diffraction grating 115 about a precisely defined pivot point 135.
During operation, optical amplifier 105 amplifies an optical field traveling along an optical axis 120. The optical field is collimated by collimating lens 110 and reflected off of diffraction grating 115. The reflected optical field recirculates back through collimating lens 110, penetrates anti-reflective coating 130, and then arrives at partially reflective mirror 125. A fraction of the optical field's power transmits through partially reflective mirror 125 and is collected as the optical output of the laser.
Diffraction grating 115 is typically mounted to a motor-actuated stage to rotate it around pivot point 135. Diffraction grating 115 acts as both a tunable filter, to coarsely select the lasing wavelength, and as a cavity mode tuner, to finely select the lasing wavelength. Diffraction grating 115 selects a different coarse wavelength by changing its angle of incidence relative to optical axis 120, and it selects a different fine wavelength through a translation that changes the length of cavity 100. Rotation about the precisely defined pivot point 135 simultaneously produces the proper amounts of angle tuning and translational tuning so that the output laser wavelength is swept in a phase-continuous way via a single actuation. An example of this design is disclosed in Trutna, W. R., and L. F. Stokes, “Continuously tuned external cavity semiconductor laser,” Journal of Lightwave Technology, v 11, n 8, 1279-1286 (1993), the disclosure of which is hereby incorporated by reference.
The design of cavity 100 is attractive in part due to its mechanical simplicity, particularly its ability to continuously tune a laser using a single mechanical motion. Nevertheless, the design of cavity 100 also has significant drawbacks.
One drawback of cavity 100 is that it has minimal error tolerance with respect to the positioning of diffraction grating 115. In particular, diffraction grating 115 must retain its precise alignment relative to pivot point 135 or the tuning mechanism will become corrupted. This can be difficult to achieve through the passage of time, changes in the device's environment, and potentially disruptive activities such as shipping. Accordingly, to avoid potential misalignments, cavity 100 generally must be built through a highly accurate, yet expensive and time consuming process.
Another drawback of cavity 100 is that the mechanical tuning mechanism tends to limit the speed of tuning. In particular, the offset pivot point 135 necessitates indirect motor drive mechanisms, which typically involve mechanical components with a great deal of inertia and low resonance frequencies, such as a mechanical level arm. These characteristics necessitate slower sweep speeds and lower repetition rates.
Yet another drawback of cavity 100 is that the required pivot point may shift as a laser is tuned over a wide spectral range. This is due in part to the cavity's inability to compensate for dispersion as the tuning occurs. Consequently, more elaborate mechanical schemes become required to maintain phase-continuous tuning over the full spectral range.
In light of these and other drawbacks of conventional technologies, there is a need for more efficient, flexible, and cost effective approaches for phase-continuous laser tuning over a broad range of wavelengths.